1. Field of the Invention
The present invention relates to a sheet processing apparatus and a sheet processing method which convey a sheet at a predetermined conveying speed.
2. Description of the Related Art
In recent years, image processing apparatuses of an electrophotographic type such as copying machines and printers can change the sheet conveying speed according to sheet types. This is to prevent deterioration of the fixability of toner on thick paper by decreasing the fixing speed because thick paper or the like has a lower toner fixability as compared with plain paper.
On the other hand, a sheet processing apparatus which receives a sheet discharged from an image forming apparatus, and carries out sheet processing such as stapling on the sheet increases the sheet conveying speed at which the received sheet is conveyed, to a predetermined speed so as to separate the sheet being conveyed from the succeeding sheet, and then reduces the sheet conveying speed to a speed suitable for discharge of the sheet onto a processing tray on which processing such as stapling is carried out. The increase/decrease in sheet conveying speed is caused by changing the rotational speed of a motor which drives rollers conveying sheets.
In the case where the above sheet processing apparatus is connected to the above described image forming apparatus which can change the sheet conveying speed according to sheet types, the conveying speed in the sheet processing apparatus must be variable within the range between the maximum speed in conveyance of plain paper and the minimum speed in conveyance of thick paper.
However, the range of conveying speeds which can be realized by one motor, i.e. the upper limit and the lower limit of the speed at which the motor is able to normally operate are limited. Therefore, even if the motor is tried to rotate at a speed outside the range, the target conveying speed cannot be reached, or even if the target conveying speed is reached, motor torque required for sheet conveyance cannot be obtained. To prevent such a situation, the use of a motor capable of running in a wide speed range is envisaged, but this would increase not only costs but also the size of the motor.
In a sheet discharging apparatus (image processing apparatus) disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H9-104555, when a sheet is discharged onto a discharged sheet tray, a gear shift means transmits the rotation of a motor to discharging rollers to cause a sheet conveyed from conveying rollers to be discharged onto the discharged sheet tray by the discharging rollers, and on the other hand, in the case where a sort means is connected in place of the discharged sheet tray to the image forming apparatus, the gear shift means switches the gear ratio to a low speed gear ratio to cause a sheet to be discharged to the sort means at a lower speed than in the case where the discharged sheet tray is connected. If this is applied, it is possible to provide a sheet processing apparatus which can accommodate a wide range of speeds without using a motor capable of running in a wide speed range.
However, to prevent a sheet from being stopped while being conveyed, it is necessary to avoid changing of gears while the sheet is being conveyed, and also, when the sheet processing apparatus discharges a sheet onto a processing tray, it is desirable that the speed is changed to a predetermined speed suitable for discharge irrespective of whether the sheet is plain paper or thick paper.